The Advanced Technology Solar Spectroscopic Imager (ATSSI) is a sounding rocket-borne experiment that will employ a Transition-Edge Sensor (TES) placed at the focus of a Wolter-I mirror to study large active region loops in the solar corona. The TES instrument will operate in the -500 - 1500 eV EUV/soft X-ray bandpass, obtain ~3 eV energy resolved spectra at ~6.25 arc-second image resolution with a count rate of ~1000 photons/sec/pixel. We will raster scan over a 0.6 x 0.6 arc minute field of view to obtain a 6 x 6 pixel image containing true EUV/soft X-ray spectroheliograms of a solar active region. Using the ATSSI observations, we can directly determine composition, electron density and thermal differential emission measure of large active region loops in order to constrain models of heating mechanisms and accurately measure the thermal morphology of these structures. In the current analysis, we present an instrument design and discuss some of the mission science goals.